Ophthalmic lenses and sunglasses have traditionally been prepared using glass as the lens material, however, plastic lenses are becoming increasingly popular due to their lightness and better shatter resistance. However, plastics are softer than glass, and therefore are inferior to glass in scratch resistance. Generally, scratch resistance and impact resistance are difficult to obtain with the same plastic (polymeric) material, so usually outer layers or coatings have been used to impart scratch resistance to an impact resistant plastic lens. A commonly used material is allyldiglycol carbonate (for example, CR-39, sold by PPG Industries) which has a high impact resistance but has a lower abrasion resistance than glass. Bisphenol-A type polycarbonates are also used since they are about two hundred times more impact resistant than glass, but are very susceptible to abrasion. Therefore, virtually all plastic lens materials, in particular, polycarbonate materials, must be coated or otherwise protected against abrasion.
An additional problem is encountered in the preparation of prescription sunglasses using polycarbonate lenses. Typically, lenses are tinted by dipping into a water bath containing an appropriate dye maintained at a temperature between about 90.degree. and 100.degree. C. The amount of tinting is related to the time of immersion in the bath. Because polycarbonate lenses cannot be tinted in this manner, a tintable coating material must be used to absorb the tinting dye. However, many of the coatings utilized which impart abrasion or scratch resistance to plastic lenses have a low absorption of dye, therefore scratch resistance is achieved at the expense of tintability of the lens.
Another technical problem dealing with plastic materials is the curing of the coating which usually involves thermal curing or radiation curing, typically by ultraviolet radiation. Coatings which require thermal curing typically involve hours for a complete cure and therefore are undesirable from a manufacturing point of view. The ultraviolet curing systems are therefore preferred in that curing requires seconds, but have the drawback of being inhibited by the presence of oxygen. For example, in U.S. Pat. No. 5,246,728, a UV curing system is disclosed requiring a photoinitiator that counteracts the effect of oxygen inhibition.
It is therefore an object of the present invention to provide an abrasion resistant radiation curable coating for plastic materials which is tintable.
It is another object of the present invention to provide abrasion resistant, radiation curable coatings for plastic materials which can be cured in the presence of oxygen or modified to be cured in the absence of oxygen.
It is a further object of the present invention to provide an abrasion resistant radiation curable tintable coating for polycarbonate materials.
These and other objects will be apparent from the following description, the appended claims from the practice of the invention.